The technical field of the invention is the field of fabricating and installing undersea production pipes for undersea extraction of oil, gas, or other soluble or fusible material, or a suspension of mineral matter, from an underwater well head in order to develop production fields installed off-shore, in the open sea, or indeed undersea pipes for injecting water or gas. The main and immediate application of the invention lies in the field of oil production, and also in reinjecting water and producing or reinjecting gas.
Undersea pipes are laid in ultra-deep sea in general from a laying ship having dynamic positioning and fitted with a J-lay tower. The tower is generally in a substantially vertical position and in its bottom portion it presents a device for gripping the pipe that extends from the laying ship down to the sea bottom where the pipe becomes naturally curved in order to take up a horizontal final position when it is resting on sea bottom, the curve being J-shaped. Such a ship is described in particular in patent WO 2000/66923.
Thus, during laying, the pipe is held securely in the bottom portion of said J-lay tower, while a new string having a length of 24 meters (m) or 48 m is installed in a horizontal position on a beam that is hinged to the structure of said tower. Said beam is then raised into a substantially vertical position by means of hydraulic actuators, and then said string is lowered to the top end of the pipe that is held in suspension in order to be accurately positioned relative to the pipe and finally welded thereto. Once welding has been completed and tested, the pipe is protected against corrosion, and then the top portion of the string is held within said tower by a gripper device secured to a carriage. The retaining device at the bottom of the tower is then released and said carriage is moved towards the bottom of the tower over a height corresponding to the length of said string. Simultaneously, the ship is moved forwards through a length corresponding to the length of string that has been lowered in this way, so as to maintain the same J-shaped curve throughout the laying operation. Finally, the locking device at the bottom of the tower is relocked, and the carriage is released and raised to the top of the tower. The cycle then begins again with a new string.
When beginning to lay an undersea pipe, successive strings are assembled together until the pipe reaches a level that is situated 10 m to 20 m above the sea bottom. The pipe is then in a vertical position. Thereafter, with the help of a cable or sling, the end of said pipe is connected to a fixed point on the sea bottom, e.g. an anchor, or indeed a pile embedded in the sea bottom, with positioning being performed by an automatic undersea remotely-operated vehicle (ROV) controlled from the surface and fitted with a manipulator arm. The ship then moves forwards while continuing to assemble new strings to the pipe, and once the J-shaped curve has been achieved, the bottom end of the pipe rests on the bottom and installation progresses in continuous manner while conserving the J-shape, i.e. while causing the ship to advance through a length that corresponds exactly to the length of the string that has just been installed.
The main drawback of that method is that once the pipe is fully installed, i.e. laid on the sea bottom, the pipe and the connecting cable and its anchor are under high traction stress. As soon as the connection cable is cut or disconnected, the end of the pipe shrinks through some variable length depending on the characteristics of the sea bed and of the pipe. It is therefore necessary to prepare special junction pipes, in general of great length, in order to ensure they have sufficient flexibility, so as to be able to make the junction with a well head or with the bottom end of a bottom-to-surface connection pipe. One such junction pipe is described in particular in patent application FR 08/58214 and in international application PCT/FR2009/052298 in the name of the Applicant. Such a bottom-to-surface connection is described in particular in patent WO 00/49267 and in numerous other patents in the name of the Applicant.
In general, the end of the undersea pipe corresponding to the end of laying, i.e. the top end of the last string, needs to have such a junction pipe connected thereto. In contrast, at the end corresponding to the beginning of laying, i.e. at the bottom end of the first string at the sea bottom, it is desired to avoid adding a junction pipe by securing the end of said undersea pipe directly to an anchor structure, thereby enabling said undersea pipe end to be positioned accurately, which end is generally fitted with a (male/female) automatic connector that is thus in position ready to be connected to the end of a second pipe fitted with a complementary (female/male) portion of said automatic connector, in particular the end of a pipe bend incorporated in an anchor structure. Numerous attempts have been carried out, but they all encounter the problem of initially forming the initial J-shaped curve under good conditions of safety, where such curving must be performed under full control. In the event of a limiting amount of curvature that is associated with the size of the pipe and with the elastic limit of steel, the pipe will be irreversibly folded or kinked, and the kinking phenomenon is generally sudden and makes the already-assembled length of pipe unusable. J-laying is not a reversible method, and if the bottom end of the pipe is damaged then the entire pipe needs to be disconnected from the ship and abandoned on the sea bottom.
More particularly, and in general, the pipe is lowered vertically as closely as possible to said anchor structure, e.g. to within 3 m to 5 m, and then a sling that is short, e.g. a few meters or 5 m to 10 m long, is connected between the bottom end of the pipe and the anchor structure. Then, by moving the laying ship, while continuing to assemble additional strings to the pipe, the J-shaped curve is formed and the end of said pipe is finally laid on a support that is secured to the anchor structure, guide elements being incorporated in said support so as to direct the connector at the end of the pipe towards its final position, which final position needs to be very accurate. The main problem is that during the approach stage, when the pipe is vertical, or even when it presents the beginning of curvature, any movement of the ship on the surface is reproduced to a greater or lesser extent at the sea bottom at the bottom end of the pipe. Pounding movements are likewise transferred in full. As a result, because of the short length of the connection cable or sling, the bottom end of the pipe runs the risk at this stage of striking the support structure for receiving it, thereby damaging the automatic connector which is a mechanical component that is complex, of high precision, and extremely expensive. In the event of damage, the complete undersea pipe is, likewise lost and needs to be disconnected from the ship and abandoned on the sea bottom. In contrast, once the J-shaped curve has been properly formed, the problem is different since the pounding movements of the ship are no longer transferred directly to the end of the pipe. The major portion of the pounding gives rise to no more than variation in the curvature of the bottom portion of the pipe, mainly in the zone corresponding to the bottom of the J-shape. As a result, risks of collision are greatly reduced, or even non-existent.
In certain documents, and in particular U.S. Pat. Nos. 5,380,129 and 3,698,199, a curving device is described for curving a pipe with the help of winches and cables, said winches being mounted on board a ship or a platform standing on the sea bottom. Those curving devices are not suitable for automatically controlling the reduction in the length of the cable and thus for automatically controlling the curvature of the pipe.
In U.S. Pat. No. 4,065,822, limiting the curvature of a pipe with the help of chains extending between two points of the pipe is described, but no means are described or suggested for tensioning, controlling, and moving the curved pipe.
In U.S. Pat. No. 3,955,599, a pipe-curving device comprises a plurality of annular segments hinged to one another and through which the pipe is caused to pass, the two end segments being connected together by an actuator. A curving device of that type is suitable for creating curvature over a short length of pipe and more particularly for creating a bend between two straight pipe fractions, however such a device does not make it possible automatically to control curvature of a greater radius of curvature that results merely from moving the bottom end of the pipe. Furthermore, the curving device described in U.S. Pat. No. 3,955,599 requires mechanical components to be installed that are relatively expensive and complex to install.